High voltage semiconductor devices commonly employ a resurf region which is a low concentration region between areas of high potential difference. The resurf region depletes as the voltage difference increases, and is fully depleted before the maximum voltage difference is applied. In the double resurf technique, there are two resurf regions of opposite polarities, both of which deplete as the applied potential difference increases. Such a device and the advantages of using the double resurf technique is detailed in U.S. Pat. No. 4,866,495.
In high voltage devices employing the double resurf technique, the resurf region of one polarity is created by implantation and diffusion of appropriate dopants into an epitaxially grown layer of opposite polarity. The epitaxial region pinched by the diffused resurf region serves as the second resurf region. High breakdown voltage is achieved when the charge in the top (diffused) resurf layer is controlled at approximately 1.times.10.sup.12 cm.sup.2 and the charge in the lower (pinched epitaxial) resurf region is controlled at approximately 1.5 to 2.times.10.sup.12 cm.sup.2. One outcome of such a construction is that as the depth of the diffused resurf layer varies slightly, the charge in the pinched epi region varies substantially so that control over breakdown voltage is lost. This effect has to be offset by using a thicker epitaxial layer. The thicker epitaxial layer has several drawbacks:
1. Deeper isolation diffusions are needed to electrically isolate different parts of the circuit on an integrated circuit from each other, requiring longer diffusion times at temperatures at or above 1200.degree. C., resulting in lower throughput.
2. Very long diffusion times at 1200.degree. C. or above, causes more defects resulting in lower yield.
3. The long diffusion times at 1200.degree. C. or above also results in a wider isolation diffusion, due to greater lateral diffusion, thus reducing the amount of useful area on a chip.